Exemplary embodiments of the present invention relate to a fuel cell system.
German patent document DE 10 2006 003 799 B4 describes a fuel cell system in which pressure is relieved from a hydrogen pressure reservoir in a turbine. This turbine then, at least partially, drives a recirculation conveying means in the region of a recirculation line around an anode chamber of at least one fuel cell of the fuel cell system. The construction, which may also have for assistance an electric motor drive for recirculation conveying means, in such case utilizes the pressure energy in the hydrogen pressure reservoir upon the expansion of the hydrogen that is necessary anyway, in order to utilize said energy for driving the recirculation conveying means.
The construction in this German patent document is comparatively complex, and is costly with regard to the sealing and the mixing of the recirculated gas flow and the fresh hydrogen. The complex structural form furthermore requires a comparatively large amount of installation space, and is correspondingly heavy. This German patent document further proposes to accelerate the hydrogen to speeds above the speed of sound of the hydrogen, i.e. to speeds above Mach 1, with the aid of a Laval nozzle. Using a Laval nozzle in such case, however, is disadvantageous because the desired effect can be achieved merely at a single load point or in the region of a single load point of the fuel cell system. For only there does the correct pressure ratio, which is dependent on the length of the Laval nozzle, occur. At all the other load points or operating points, what is called a vertical compression shock will occur. After the Laval nozzle, speeds that are lower than the speed of sound then occur.
German patent document DE 10 2008 045 170 A1 describes the utilization of the heat sink produced upon the expansion of hydrogen from a hydrogen pressure reservoir in order to cool heat generating component parts in the fuel cell system.
German Utility Model DE 20 2005 017 574 U1 discloses a split cage motor for driving a recirculation conveying means for recirculating anode exhaust gas around the anode chamber of a fuel cell. The advantage of such a split cage motor is that the hydrogen carrying region together with the rotor can be sealed off hermetically with respect to the stator of the split cage motor, so that there are no complications with regard to the sealing off of hydrogen carrying regions.
Exemplary embodiments of the present invention are directed to a very compact and energy efficient fuel cell system that avoids the disadvantages mentioned above.
According to exemplary embodiments of the present invention, a compressor wheel and turbine are combined in a single component so that a simple and efficient fuel cell can be provided that has a very compact construction. Thus, in addition to the high energy efficiency due to the utilization of the pressure of the hydrogen after the pressure regulator of the compressed gas reservoir, a very compact construction of the device is also achieved. This can be made correspondingly simple, lightweight and with a low structural volume. In addition to the structural volume, this also saves on weight and costs.
In one particularly beneficial development of the fuel cell system according to the invention, the hydrogen flows onto the turbine via a nozzle having a nozzle needle, the throughflow of hydrogen being adjustable by a relative movement of the nozzle with respect to the nozzle needle. The throughflow of hydrogen through the nozzle and the displaceable nozzle needle—or conversely—is adjustable, and can thus be adapted to the respective load demand or the respective operating point of the fuel cell system. In order to achieve the highest possible speed at the nozzle exit, the annular area produced by the needle and nozzle should be the narrowest point of the hydrogen metering device. If at the same time a critical pressure ratio is present, then, driven by this pressure ratio, the speed of sound of the hydrogen is achieved at the narrowest point of the needle valve. The hydrogen then enters the turbine in ideal manner at Mach 1.
In a further very advantageous configuration of the fuel cell system according to the invention, the turbine is a Pelton turbine. Such a Pelton turbine utilizes the energy applied by a gas jet, which can be generated, for example, in accordance with the configuration set forth above via a nozzle, comparatively efficiently, so that a particularly high energy yield can be achieved with the Pelton turbine. Thus, the energy requirement that has to be applied by other drive means can be minimized, so that overall the efficiency of the fuel cell system can be increased.
In a further very beneficial and advantageous variant of the fuel cell system according to the invention, the component has channels connecting the region of the turbine and the region of the compressor wheel together. In addition to the pure momentum exchange between the hydrogen gas under high pressure that is expanding and the turbine, overflowing of gas out of the region of the turbine thus also takes place once it has imparted a momentum to the turbine, in the region of the compressor wheel. Thus, the volumetric flow of the recirculated exhaust that is conveyed via the compressor wheel is ideally mixed with the volumetric flow of the fresh hydrogen penetrating via the channels out of the region of the turbine into the region of the compressor wheel, so that a very well mixed and very homogeneous gas flow of fresh hydrogen and recirculated anode exhaust gas is present after the recirculation conveying means. This means that the route between the recirculation conveying means and the anode chamber of the fuel cell can be shortened, since it no longer has to be used in order to ensure the uniform distribution of the gas. Accordingly, installation space and material for lines can be saved with the fuel cell system. In addition to the saving on installation space, a saving in terms of weight and costs also occurs.
In a beneficial configuration of the invention, the channels are formed, starting from the region of the turbine, such that the hydrogen flowing through the channels flows into the region of the compressor wheel substantially in the same direction as the flow of the exhaust gas in the compressor wheel. In the case of a configuration of the compressor wheel as a momentum exchange machine or side channel compressor, it is thus achieved that a momentum of the hydrogen flowing through the channels due to the inflow in the same direction as the conveyed exhaust can be utilized such that said hydrogen supports the vortical movement of the exhaust gas through the clever configuration of the channels. Due to the fact that the natural vortical movement of the exhaust gas is supported by the inflowing fresh hydrogen, the efficiency of the compressor wheel can be increased.
In an advantageous development of the fuel cell system according to the invention, the component can be driven by means of an electric drive motor, the drive motor being formed as a pancake motor. Typically, it may be necessary for a further drive to be present in addition to the drive of the recirculation conveying means via the turbine, in order for example to be able to continue to operate the recirculation conveying means even when the fuel cell system is idling, in order thus to ensure the uniform distribution of the gases in the recirculation circuit. Furthermore, in certain operating states the energy in the region of the turbine alone is not sufficient to drive the recirculation conveying means due to the admetered amount of fresh hydrogen being very small. Then the electric drive motor can contribute to driving the component, and hence in particular the compressor wheel. The drive motor in this case should be formed as a pancake motor. Such a pancake motor is constructed similarly to a split cage motor, and has a stator and a rotor that is formed to be able to be sealed off hermetically therefrom. Unlike with a split cage motor, however, it has a flat structural form, so that the rotor is arranged as a disk-shaped component spaced apart from the stator, which substantially likewise is formed in a disk shape. The construction thus, with the necessary sealing between rotor and stator, which in the case of a hydrogen atmosphere would be practically unable to be realized by seals with rotating union, permits a very compact structural form. The arrangement of the rotor relative to the stator in a sealed-off space is also referred to as a plate seal.
In a particularly beneficial development of this aspect, a stator of the pancake motor can be arranged above the component, the component being provided with magnetic elements and forming a rotor of the pancake motor. This can, for example, be realized by introducing additional magnets into the component, which magnets point with their magnetic fields in the direction of the stator of the pancake motor and are sealed off therefrom by a wall which is magnetically permeable. Corresponding excitation of the stator then ensures rotation of the rotor and hence rotation of the component, so that rotation of the compressor wheel also takes place.
In a beneficial development of this aspect, the rotor can be arranged in a space which is sealed off relative to the stator. This construction, as is conventional with pancake motors or split cage motors, is particularly advantageous when used with the hydrogen atmosphere, since the hydrogen atmosphere can then be sealed off comparatively simply, since there are no rotating unions through seals or similar.
In a particularly beneficial and advantageous development thereof, the space can be the interior of a water separator in the recirculation line. The construction, and thereof in particular the component with the turbine and the compressor wheel, can therefore be integrated directly in a water separator, for example by the latter forming the cover of such a water separator. The stator of the pancake motor is then arranged above the water separator, so that overall a very compact construction is produced that integrates the recirculation conveying means into a water separator which is present anyway and manages with minimal additional installation space compared with a conventional water separator.
As is also known from the prior art, the bearings of the component may be formed at least partially as gas bearings or hydrogen bearings, which are operated by hydrogen from the region of the hydrogen pressure reservoir. In addition, according to an advantageous configuration of the fuel cell system according to the invention, provision is made for at least one of the axial bearings of the component to be formed by means of a bearing tip. Such a bearing by means of a bearing tip relies on minimization of the friction by a pointed or spherical shaping of a bearing element, which then runs in a corresponding shell or ideally, in the case of a spherical formation, on a corresponding counter tip with a spherical surface. Such a construction can be realized here too, since it provides a very simple and efficient axial bearing with minimal effort and minimal costs.
In this case, the bearing tip can be formed spherically and to run on a correspondingly spherical counter surface. One of the surfaces may in this case be formed of ceramic, and the other surface of steel. With time, at most the steel tip will wear away, but this can be replaced very simply. The bearing may, for example, be braced via spring elements or the like such that the tips or spheres always contact one another with a comparable contact pressure force in regular operation. Further, the region of greatest Hertzian stress in the region of the sphere (i.e. the region in which the frictional heat is produced) can be cooled via a volumetric hydrogen flow.
An exemplary method in accordance with the present invention, the electrical components and/or magnetic components and/or bearings in the region of the recirculation conveying means can be cooled by means of hydrogen. Since the fundamental part of the recirculation conveying means with bearings and other heat generating components is typically the region around the component, here there is the possibility of achieving particularly simple and efficient cooling by cooling with hydrogen, since the hydrogen from the tank is approximately at ambient temperature and thus will be significantly cooler than the hydrogen in the anode circuit. The latter can therefore cool the components without additional cooling media having to be brought into the hydrogen carrying region, with corresponding expense with regard to sealing. A further advantage of this method is that the hydrogen is heated up thereby. This results in a reduction in the condensation of water if the hydrogen meets the recirculated exhaust of the anode chamber. This reduces, or ideally avoids entirely, the introduction of liquid into the anode chamber, so that the performance of the fuel cell is not adversely affected by channels “blocked” with liquid water in the anode chamber.
In accordance with exemplary embodiments of the present invention, the method can involve setting the throughflow through the nozzle to such that a speed of the hydrogen upon entry into the turbine is in the range from 0.8-1.05 of the speed of sound (Mach 1) of the hydrogen. Such a flow rate of the hydrogen guarantees a high momentum exchange onto the turbine, which may in particular be formed as a Pelton turbine, and also makes it possible to effect controlled imparting of the momentum onto the turbine, since ultrasonic flow effects (compression shocks) typically do not occur here.
In a particularly beneficial method for driving a recirculation conveying means in a fuel cell system the electric drive motor can be designed for a constant or approximately constant operating point. This designing of the electric drive motor for a constant or approximately constant operating point and the variation of the drive power required for the compressor wheel by a corresponding variation of the hydrogen flow onto the turbine makes it possible to use a very simple and inexpensive electric drive motor with correspondingly simple and inexpensive electronics, since said motor, due to the constant or approximately constant operating point for which it is designed, can be realized correspondingly simply and efficiently and only a low mechanical drive power has to be provided. A large spread of speeds can be dispensed with, so that as a result of this, in addition to an improvement in the efficiency, a reduction in costs, installation space and weight can also be achieved. The electric drive motor as a result can preferably be designed as a low voltage motor.